1. Field of the Invention
The invention is directed to a hydraulic cylinder including a housing having a fluid connection for connecting to a hydraulic system and an after-running connection for connecting to a fluid after running vessel, a piston which is axially displaceable in the housing to define a variable pressure space, a seal isolating the pressure space from the atmosphere, and an after-running groove which bridges the seal when the piston is in an extended end position.
2. Description of the Related Art
A hydraulic cylinder of the type mentioned above, such as is applied, for example, in automotive engineering in hydraulic clutch systems or brake actuation systems, is known, e.g., from U.S. Pat. No. 6,584,771.
The housing of the hydraulic cylinder shown in the above-cited publication has, at its end side, a connection for a pressure line and, at its jacket surface, a connection with an after-running channel extending therein for connecting to an after-running vessel. A separate, ring-shaped after-running device is arranged at the end side of the cylinder piston facing the pressure space and twelve axial after-running grooves (snifting grooves) are uniformly distributed at the outer and inner circumferential surfaces of this after-running device so as to cooperate jointly with the after-running channel, an after-running area and a primary seal arranged in the cylinder.
With the piston in its extended end position in which the pressure space has its maximum volume, the after-running grooves bridge the seal which is fastened in the housing and which contacts the piston. Accordingly, on the one hand, hydraulic fluid can flow from the after-running vessel into the pressure space and, on the other hand, gas bubbles that have collected in the venting groove can escape through the after-running channel. When the piston is displaced in the direction of the pressure space, fluid that has been displaced from the pressure space will be conveyed into the after-running channel again as long as the fluid connection via the after-running grooves is open, so that gas bubbles still located in the pressure space can also escape. In this way, the pressure space of the hydraulic cylinder is vented automatically with each stroke.
In order to improve the venting behavior, DE 199 53 286 A1 proposes a hydraulic cylinder with after-running grooves that are arranged at the piston and a venting groove that is formed at the cylinder housing along the inside of the pressure space. The base of the venting groove slopes upward toward the venting channel or after-running channel relative to a horizontal of the intended installation position of the cylinder. Due to this construction, gas bubbles located in the pressure space can collect in the geodetically highest venting groove and are simply guided out of the area near the end position of the piston via the venting channel or after-running channel.
It is disadvantageous in the known hydraulic cylinders on the whole that, in spite of the plurality of compensating grooves, a venting of the pressure space is nevertheless inadequate when installed horizontally as is usual. As a result, a portion of the gases that have collected in the venting groove remains in the pressure space and, when the piston is actuated accompanied by a reduction of the pressure space, these gases are compressed and accordingly cause an undesirably long idle path and dead time in the hydraulic actuation system.
DE 199 15 832 A1 discloses a hydraulic cylinder with a piston travel sensor. The piston-cylinder unit, which is a master cylinder in this example, is activated by means of a brake pedal or clutch pedal that communicates with the piston rod of the cylinder. The hydraulic pressure generated in the master cylinder is transmitted via a line system filled with hydraulic fluid to a slave cylinder which causes a displacement of a working piston and, in this way, can actuate a clutch release, for example. Further, the piston-cylinder unit mentioned above has a sensor system which serves to detect the position of the piston inside the cylinder. For this purpose, a ring groove which is open toward the inner wall of the cylinder is formed at the outer circumference of the piston and a ring-shaped permanent magnet is arranged at the ring groove and is axially displaceable with the piston along the inner wall of the cylinder. Fastened to the outer wall of the cylinder housing is a receiving part in which are arranged two Hall switches whose axial distance corresponds to a starting position and end position of the piston and which respond to the magnetic field of the permanent magnets located at the piston and trigger a switching process or an electric signal when the permanent magnet passes by. The receiving part has an electric connection contact part for conveying the signal to control structures.
The above-cited publication does not specify how the permanent magnet is fastened to the piston. However, judging from the drawing, it can be assumed that the piston is manufactured in a first work step and the ring magnet is slid into the ring groove of the piston from the side of the piston remote of the pressure space in another work step. It is disadvantageous that even with the greatest care it cannot be ruled out in this assembly of the piston and permanent magnet that the permanent magnet will subsequently detach from its mounted position at the piston due to sudden, shock-like loading of the piston so that detection of the position of the piston cannot carried out successfully. In extreme cases, the permanent magnet may burst resulting in blockage of the piston and therefore in failure of the hydraulic system. It is apparent in the sensor system that the Hall switches are only arranged at a circumferential position, while the permanent magnet is symmetric with respect to rotation, which, because the piston can freely rotate about its longitudinal axis, is usually carried out in this way so that the piston can generate a signal in every rotational position.